According to Encyclopædia Britannica and others, the Shia are believed to have started as a political party and developed into a religious movement, influencing Sunnis and produced a number of important sects. Early in the history of Islam, the Shīʿites were a political faction (Arabic shīʿat ʿAlī, “party of ʿAlī”) that supported the power of ʿAlī ibn Abī Ṭālib (the fourth caliph [khalīfah, successor of Muhammad]) and, later, of his descendants.
Twelver Shia believe in the lineage of the Twelve Imams. The Twelver Shia faith is predominantly found in Iran (est. 90%), Azerbaijan (est. 65%), Bahrain (est. 70%), Iraq (est. 60%), Lebanon (est. 24%), Kuwait (est. 33%), Turkey (est. 15%), Albania (est. 10%), Pakistan (est. 10–15%) and Afghanistan (est. 15%).The Zaidi Shia are predominantly found in Yemen (est. 40%).
The Zaidi dispute the succession of the fifth Twelver Imam, Muhammad al-Baqir, because he did not stage a revolution against the corrupt government, unlike Zayd ibn Ali. They do not believe in a direct lineage, but rather that any descendant of Hasan ibn Ali or Hussein ibn Ali who stages a revolution against a corrupt government is an imam.
The Ismaili dispute the succession of the seventhi imam, Musa al-Kadhim, believing his older brother Isma'il ibn Jafar actually succeeded their father Ja'far al-Sadiq.
Ismailis believe that Ja'far al-Sadiq thought his son, Isma'il ibn Jafar "al-Mubārak", would be heir to the Imamate. However, Isma'il predeceased his father. Some of the Shia claimed Isma'il had not died, but rather gone into occultation, but the proto-Isma'il group accepted his death and therefore that his eldest son, Muhammad ibn Ismail, was now Imam. Muhammad remained in contact with this "Mubārakiyyah" group, most of whom resided in Kufah. Ismailis are dominant group in Badakhshan. They form small communities in Afghanistan, Pakistan, Uzbekistan, India, Yemen, China and Saudi Arabia and have several subbranches.Twelver Shia or the Ithnā'ashariyyah' is the largest branch of Shia Islam, and the term Shia Muslim usually refers to Twelver Shia Muslims only. The term Twelver is derived from the doctrine of believing in twelve divinely ordained leaders, known as The Twelve Imams. Twelver Shia are also known as Imami or Ja'fari, originated from the 6th Imams name, Ja'far al-Sadiq, who elaborated the twelver jurisprudence.
Twelver doctrine is based on five principles. These five priciples known as Usul ad-Din , which appears to be simple but in elaboration and practice go far off from the original teachings of Islam. They are as follow:
- Monotheism, God is one and unique.
- Justice, the concept of moral rightness based on ethics, fairness, and equity, along with the punishment of the breach of said ethics.
- Last Judgment, God's final assessment of humanity.
- Prophethood, the institution by which God sends emissaries, or prophets, to guide mankind.
- Leadership, A divine institution which succeeded the institution of Prophethood. Its appointees (imams) are divinely appointed.